This invention relates to quality control and reliability in the automated fabrication or manufacture of nuclear fuel materials. These materials can be used in nuclear reactors for power production.
Such quality control promotes the operational reliability of nuclear fuels. It is important to qualitatively and quantitatively determine the amounts and distribution of the various materials which can be combined to constitute an amount of nuclear fuel. Of particular interest is to obtain the neutron absorber profile of nuclear fuel rods to maintain quality control over neutron absorber materials.
These fuel rods are generally tube-like with the ends suitably plugged to contain, among other things, right cylindrical nuclear fuel pellets. A fuel rod may include nuclear fuel materials in suitable form such as a powder or pellets. These are enclosed within a corrosion-resistant, non-reactive cladding which is sealed on each end with an end plug, usually of the same composition as the cladding. Such fuel rods are generally described in U.S. Pat. Nos. 3,382,153; 3,349,004; and 3,741,768 (expressly incorporated herein and assigned in each case to General Electric Company). These fuel rods have been referred to by other names including "fuel elements" or "fuel pins" in the literature. Insofar as the term fuel rods is employed herein, the term is intended to apply to fuel elements and pins as well.
Fuel rods may, for example, contain about 3.3 kg of uranium dioxide pellets sealed in a zirconium alloy tube. A plurality of such fuel rods may be grouped together at fixed distances from each other in a coolant flow channel or region as a fuel assembly and typically a sufficient number of fuel assemblies is combined to form a nuclear reactor core capable of a self-sustained fission reaction, as is described in Nuclear Power Engineering by M. M. Eli-Wakil, published by McGraw-Hill Book Company In 1962. Further information in this regard may be found in a volume edited by D. M. Considine: Energy Technology Handbook (McGraw-Hill, 1977).
Furthermore, in neutron-absorber-type nuclear fuels, the neutron absorber material mixed with fissile uranium is gadolinium. The magnetic susceptibilities of uranium and of gadolinium ions are significantly different. Accordingly, it is possible to measure magnetic susceptibility as a function of length along selected fuel rods by passing them through inductive detector coils in magnetic fields at constant velocity and measuring certain response signals. This permits the accurate measurement of the distribution of additives such as gadolinium in nuclear fuel rods by magnetic measurement techniques.
Uranium is commonly used in various forms as a reactor fuel, for reasons of both availability and use. It can be employed in the form of a pure metal, as a constituent of an alloy, as an oxide, carbide, nitride, or other suitable compound.
In the embodiment herein, a ceramic fuel of enriched uranium dioxide is employed. In manufacturing production, the uranium dioxide is ground into a fine powder and compacted by cold pressing into small cylindrical pellets and then sintered in a neutral or reducing atmosphere and thereby increased in density. The pellets are ground to specific dimensions and are loaded into long, thin zircaloy tubes which serve as cladding. When the tubes are fully loaded, an expansion spring is inserted at the top, the space (plenum) is filled with helium, and finally an end cap is suitably sealed or welded in place. Inspection of the fuel pellets after grinding, of the zircaloy tubing during fabrication, and of the completed fuel rod (or fuel pin) provides assurance that all specifications are being met. Rods which pass the inspection tests are then assembled into bundles, with appropriate spacers according to reactor type.
The materials selected for nuclear fuel use according to the instant invention include fissile and fertile materials and certain neutron absorbing additives. Suitable neutron absorber materials, specifically "burnable" neutron absorber materials include, for example, gadolinium and europium. The Naval Reactors Physics Handbook, Vol. I, (AEC, 1964), for example, includes at pg. 817 the following list of materials potentially useful as burnable absorbers (i.e., poisons): boron, silver, cadmium, indium, samarium, europium, gadolinium, dysprosium, hafnium, lithium, erbium, iridium, and mercury. The gadolinium employed in the instant embodiment is preferably in oxide form, e.g. Gd.sub.2 O.sub.3 (gadolinia). The reason for employing burnable absorbers in thermal reactors (as opposed to fast, or breeder reactors) is to compensate for the depletion of neutrons of fissile material in the fuel and an initial excess reactivity in the core in order to provide a reasonable core operation period between refueling episodes. Additionally, the nuclear fuel rods include zircaloy and steel for the construction of the tube cladding and the plenum spring respectively.
In a nuclear fuel material comprising either an elemental or compound form of uranium, plutonium, thorium or mixtures of the foregoing, various additives have been suggested to impart particular properties to the nuclear fuel material during nuclear fission chain reactions or to aid in the preparation of the nuclear fuel materials for use in nuclear reactors. Where these additives have an appropriate magnetic susceptibility or paramagnetic character, it is possible to use the magnetic response to detect the additive and to determine the amount of additive present in a nuclear fuel material, either alone or as contained in a cladding, since the nuclear fuel material and the cladding commonly used have a very low magnetic susceptibility. Representative of the additives having suitable magnetic properties which can be readily detected by the practice of the method of this invention are burnable absorber materials of gadolinium, dysprosium, europium, or erbium in elemental and compound form including respectively gadolinium oxide (Gd.sub.2 O.sub.3), dysprosium oxide (Dy.sub.2 O.sub.3), europium oxide (Eu.sub.2 O.sub.3), or erbium oxide (Er.sub.2 O.sub.3). Other additives that can be detected in elemental or compound form by the method of this invention include iron, nickel, manganese, holmium, cobalt, terbium, and thulium. Plutonium is detectable by this method when it is the only additives having any appreciable magnetic susceptibility in a nuclear fuel material.
Magnetically, UO.sub.2 and Gd.sub.2 O.sub.3 are paramagnetic and have susceptibilities, X, of 8.74.times.10.sup.-6 emu/g-Oe and 147.times.10.sup.-6 emu/g-Oe, respectively. The processing of fuel pellets for use in reactors typically introduces up to 500 ppm of elemental iron and/or ferromagnetic alloys as impurities or inclusions.
Several of the burnable absorber additives which are incorporated in nuclear fuel materials exhibit a magnetic susceptibility which is in marked constrast to the magnetic susceptibility of the other components forming the zircaloy-fuel elements including for example uranium, plutonium, and Zircaloy-2 cladding material.
It is known that the magnetic susceptibility of additives in nuclear fuel materials in comparison to the magnetic susceptibility of the nuclear fuel material and the cladding in which the nuclear fuel material is contained permits the reliable non-destructive qualitative and quantitative detection of these additives in nuclear fuel materials.
One such technique for such additive measurement is outlined in U.S. Pat. No. 4,243,939 which issued on Jan. 6, 1981 (to patentees Grossman, Portis, Bernatowicz, and Schoenig) and is assigned to General Electric Company. This patent is hereby expressly referred to an incorporated herein. Other patents of possible general interest in the subject area of magnetic detection in the nuclear arts are U.S. Pat. No. 3,787,761 (Grossman and Packard) and U.S. Pat. No. 4,134,064 (Jacobs, Lahut, and Grossman), each of them assigned to General Electric Company.
Gadolinium-bearing absorber rod designs in current use and in production since 1977 may contain zones of nominally pure UO.sub.2 at both ends of the fuel rod, and zones containing (U,Gd)O.sub.2 between the pure UO.sub.2. These fuel rod designs are commonly called "winged" designs, and are so referred to herein. Fuel rod designs called "non-winged" designs contain no nominally pure UO.sub.2 zones at either or both ends.
Assays of fuel rods can be accomplished with a satisfactory degree of precision according to the Grossman et al. technique stated in the '939 patent with respect to the winged fuel rod construction. However, the results are not equally satisfactory for non-winged designs, as shown in Table I below. The Table is stated in terms of uncertainty, which is an inverse measure of precision.
TABLE I ______________________________________ FUEL ROD GADOLINIA ASSAY UNCERTAINTY AT THREE TIMES VARIANCE FOR ZONE WEIGHT PERCENT GADOLINIA A B C Former Method Former Method Present Invention For Winged Rods For Non-Winged Rods For Either Case ______________________________________ .+-.0.18 wt. % .+-.0.6 wt. % .+-.0.10 wt. % ______________________________________
The precision of measurements as shown in Table I are accordingly significantly better for winged rather than non-winged rods according to the former method of quality control measurement for gadolinium or other neutron absorber content. This is considered to be the result of drift problems in the electronic circuitry making the measurements, which are resolved (see Col. C) in the instant invention. The uncertainties presented in Table I are three times the variance in measurement of weight percent gadolinia per zone of fuel column. A zone length is typically about a meter, and minimum zone lengths are typically about 15 cm of fuel column. The data in Table I relating to the former method are based on the use of the nominally pure UO.sub.2 wings as "internal standards"; in other words, the gadolinia content of the fuel column zones is referenced to the gadolinia-free wings. In this manner, i.e., by referencing to the wings, the measurement of susceptibility changes beyond the fuel column length is avoided. This avoidance is especially significant under the former method in view of the large susceptibility changes occurring, which are caused by the presence of ferromagnetic components such as the ferromagnetic spring in the plenum region of a fuel element. Susceptibility changes of this magnitude are hazardous to the measuring devices or circuitry employed formerly.
Drift problems may be the result of many factors including noise, interference, or thermally induced electro-motive-forces. The former measurement circuitry also has limited dynamic range with respect to input signal variations. The requirement for adequate signal resolution drives the former circuitry into saturation when large signal variations from ferromagnetic susceptibility changes occur, e.g., when the mass of a ferromagnetic spring of the plenum region of the fuel rod is passed through the measuring circuitry. The susceptibility variations ranging from air (which has a susceptibility near zero) to masses of material such as the end plugs, thermal barriers, and plenum springs within the fuel rod, are simply too great to be adequately handled with circuitry of limited dynamic range.
The drawing introduced below provides an indication of the kinds of signal variations induced in an inductive detector coil according to the Grossman technique stated in the '939 patent, when measuring a typical fuel rod. Further, a brief structural description of the fuel rod is provided.
The nuclear fuel industry has long required assurances of high quality and reliable fabrication of nuclear fuel for power production. Accomplishing this objective has caused manufacturing performance to improve in recent years and has upgraded capabilities for quality detection and control of nuclear fuel. Among other things, a computerized or microprocessor-based and controlled system for automated data acquisition and retention of fuel information has been developed.